The present invention generally relates to motor control methods and disk units, and more particularly to a motor control method for stopping rotation of a motor by applying to the motor which is rotating in a forward direction a brake signal which rotates the motor in a reverse direction, and to a disk unit which employs such a motor control method.
As methods of stopping the rotation of the motor, there is a short-circuit brake method, a reverse rotation brake method and the like. The short-circuit brake method stops the rotation of the motor by short-circuiting motor windings. This short-circuit brake method is simple, but it takes a relatively long time until the motor stops completely. On the other hand, the reverse rotation brake method stops the rotation of the motor by applying to the motor which is rotating in the forward direction a brake signal which rotates the motor in the reverse direction. Hence, the reverse rotation brake method can stop the motor completely within a short time.
In disk units such as an optical disk unit which is designed so that a disk is loaded into and unloaded from the disk unit, an eject operation is carried out when unloading the disk from the disk unit. When this eject operation is carried out, a spindle motor which rotates the disk is stopped, and the disk is ejected from the disk unit in a state where the disk has completely stopped rotating, so as to protect the disk. When the operation characteristic of the disk unit is taken into consideration, it is desirable that the disk is ejected as quickly as possible from the disk unit when the eject operation is carried out. For this reason, it is desirable in the case of the disk unit to employ the reverse rotation brake method which can stop the rotation of the spindle motor within a short time.
When controlling the rotation of the spindle motor, this rotation control is greatly affected by environmental conditions of the spindle motor, that is, the environment in which the disk unit is used. When a constant voltage drive type integrated circuit (IC) which is popularly used is employed to apply a current to the spindle motor, this current depends upon a power supply voltage of the disk unit, and the current varies similarly to the power supply voltage when the power supply voltage varies. In addition, a friction of a shaft of the spindle motor depends upon a temperature within the disk unit due to a temperature characteristic or the like of a lubricant coated on the shaft portion, and thus, the friction of the shaft varies as the temperature varies.
Accordingly, even if an optimum value of a brake time which is obtained with respect to the spindle motor under conditions in which the power supply voltage does not vary is used, the spindle motor which is rotating in the forward direction will start rotating in the reverse direction before the brake time ends when the power supply voltage has an upper limit value within a tolerable range and the constant voltage drive type IC is employed. In this case, the spindle motor will continue to rotate in the reverse direction for a while due to inertia even after the brake time ends. On the other hand, when the power supply voltage has a lower limit value within the tolerable range, the spindle motor which rotates in the forward direction will continue to rotate in the forward direction for a while even after the brake time ends. In addition, even if the optimum value of the brake time which is obtained with respect to the spindle motor under room temperature is used, the spindle motor will start rotating in the reverse direction before the brake time ends under low temperature conditions, and the spindle motor will continue to rotate in the reverse direction for a while due to inertia even after the brake time ends. On the other hand, when the power supply voltage has the lower limit value within the tolerable range, the spindle motor which is rotating in the forward direction will continue to rotate in the forward direction for a while under high temperature conditions even after the brake time ends.
In order to prevent rotation of the spindle motor due to inertia after the brake time ends, it is conceivable to provide a motor rotation detection means for detecting the complete stop of the spindle motor. However, in this conceivable case, it becomes necessary to provide Hall elements or the like as the motor rotation detection means, in addition to a motor control circuit, and a control system for the spindle motor would become complex and expensive.
Conventionally, the motor control method does not take into consideration the environment in which the motor is used. For this reason, if the power supply voltage or the temperature varies, there was a problem in that the motor will not come to a complete stop even after the brake time ends.
In addition, although it is conceivable to provide a motor rotation detection means for detecting the complete stop of the motor, the control system for the motor would become complex and expensive. Hence, it is desirable to accurately and positively stop the motor within a short time without the need to provide a motor rotation detection means.
It is conceivable to generate a pulse signal when the spindle motor rotates by a relatively simple circuit within a control circuit which rotates the spindle motor by applying a driving current having three phases, for example. This pulse signal can be generated by detecting a counterelectromotive voltage of the spindle motor and binarizing the counterelectromotive voltage waveform. However, when a ratio L/R between an inductance L and a resistance R of the coil of the spindle motor is large, the pulse signal will become irregular as the rotational speed of the spindle motor becomes small. But since the magnetic force must be set high due to the small capacity when the start time is to be reduced particularly in the case of a thin type spindle motor, this ratio L/R becomes a relatively large value.
FIG. 1 is a diagram showing the signal waveforms of the pulse signal P described above and a signal B which instructs the start of the braking. In addition, FIGS. 2, 3 and 4 respectively are diagrams showing the signal waveforms of the pulse signal P at portions P1, P2 and P3 shown in FIG. 1 on an enlarged scale. In FIGS. 1 through 4, the ordinate indicates the signal level in arbitrary units, and the abscissa indicates the time in arbitrary units. It may be seen from FIG. 3 that the pulse signal P becomes irregular particularly from a portion D in FIG. 3 where the rotational speed of the spindle motor becomes small.
Therefore, although it is conceivable to detect the stop of the spindle motor by use of the pulse signal P described above, it may be seen from the irregularity generated in the pulse signal P at the low rotational speeds of the spindle motor that this conceivable method is not practical.